MIPS: Remove platform-specific dead code for KeyedStores

Port r16227 (02df4098)

BUG=

Review URL: https://codereview.chromium.org/22815014

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@16251 ce2b1a6d-e550-0410-aec6-3dcde31c8c00